1. Field of the Invention
The present invention relates to a motor used in a linear actuator, and in particular to a motor having a braking function and used in a linear actuator.
2. Description of Prior Art
Linear actuator employs a motor, a worm rod, a worm wheel and other driving elements to drive a lead screw to rotate. The lead screw drives a telescope tube threadedly connected onto the lead screw to generate a linear extending action or retracting action. Since the structure and installation of the linear actuator are easier and more convenient than those of a pneumatic cylinder or a hydraulic cylinder, the linear actuator has been widely used in a sickbed, electric chair or other places where extending and retracting actions are needed.
EP0662573B1 and WO2004-027290A2 disclose a linear actuator respectively. The linear actuator disclosed in EP0662573B1 includes a worm rod, a lead screw, a worm wheel set fixed to one end of the lead screw. The worm wheel set comprises a worm wheel. A helical spring surrounds the outer periphery of a cylindrical ring. In this way, the linear actuator can be obtained. In view of the problems of the EP0662573B1, the linear actuator disclosed in WO2004-027290A2 is provided with a heat-conducting element for dissipating the heat generated by the frictional force between the helical spring and the cylindrical ring.
However, in the conventional linear actuator, the helical spring is stationary, but the cylindrical ring is rotatable together with the worm wheel. Thus, heat is generated due to the friction force between the helical spring and the cylindrical ring. The cylindrical ring is made of plastic materials. The heat generated by the frictional force makes the temperature of the cylindrical ring to increase and even melt. Or, the helical spring may be worn quickly to reduce its lifetime. Only a line contact is formed between the helical spring and the cylindrical ring to generate a decelerating effect or braking effect. Thus, the contact area between the helical spring and the cylindrical ring is not large enough to generate a sufficient decelerating effect or braking effect.